The present invention relates in general to semiconductor laser structures which incorporate strain-compensated multiple quantum wells as the laser gain medium.
High speed surface emitting lasers (SELs) are useful in optical communications, optical interconnects and many military applications. One very attractive method of making surface emitting lasers is to use the mechanism of surface grating coupling. The distributed feedback (DFB) laser and the distributed Bragg reflected (DBR) laser are two examples of such lasers which employ first or second order gratings to couple laser light from a horizontal cavity vertically through a horizontal surface of the laser. When a guided light wave "sees" second order gratings (a portion of the optical field of the light wave intersects the gratings), the light will simultaneously experience both a reflection in the opposite direction and diffraction normal to its propagation axis. The reflection can be used to create the optical feedback required for a laser diode, while the diffractions can be used for surface emission. First order gratings are often employed in combination with the second order gratings because the first order grating is more efficient in reflecting the light than is the second order grating. However, the first order grating does not produce surface coupling. Therefore, it is sometimes favorable to combine first order gratings with second order gratings to achieve both efficient feedback and surface coupling.
DFB lasers have their grating regions extended along almost the entire laser cavity, while DBR lasers have their grating regions formed at the ends of the laser body. The gratings of DFB lasers are normally located below and above the laser's active region, so that light created by the active region "sees" the gratings immediately. In contrast, light generated in the active region of a DBR laser "sees" no gratings until near the ends of the laser body, and there is no active layer in the grating region. The characteristics of DFB and DBR lasers are in many ways comparable except for some subtle differences.
DFB lasers are currently more popular and more successful commercially because their fabrication process, though expensive and complicated, is not as expensive and complicated as that of DBR lasers. The key to reducing their cost is to simplify the fabrication and packaging costs. The former involves the improvement of device process, testing, throughput and yield, while the latter involves simpler optical alignment and assembly. Since surface emitting lasers can be batch fabricated, on-wafer tested and are much more easily adapted for device/optical fiber coupling, grating coupled surface emitting lasers have long been considered as a promising candidate for low cost DBR/DFB type lasers. However, several problems remain that limit the use of grating coupled DFB/DBR lasers in commercial applications.
For DBR type grating coupled surface emitting lasers, there is a trade off between coupling efficiency and laser performance. If one wants to achieve high performance (e.g., low threshold current), the DBR mirror is preferably formed with both first and second order gratings so that the reflectivity can be made high. This results in a lower power output since the light intensity decays quickly in the first order gratings and not much light can be coupled vertically through the laser's top surface by the second order gratings. On the other hand, if second order gratings are connected directly to the gain region, the mirror loss will be too high to achieve a low threshold current. In both configurations, the output intensity is an one-sided exponential decay function that does not match the mode field of the optical fiber to which the laser is coupled, thereby leading to a low coupling efficiency.
For DFB lasers having second order gratings at the center or throughout the entire laser cavity, the output beam spot in the laser axis will be hundreds of microns long, and the net output power in the surface normal direction will be very low. This is because the surface coupling has to be weak or the device will suffer from high coupling and scattering loss. It should be noted that surface coupling can be considered as optical loss since it removes photons from the laser cavity. The exceedingly large laser beam spot size, in spite of its narrow diffraction angle, makes the optical coupling to single mode fibers difficult without sophisticated lenses. Finally, because of the weak reflective and surface coupling by first or second order gratings, the overall device size cannot be made smaller than a few hundred microns, and this limits the device fabrication throughput.
The parameter that characterizes the coupling strength or efficiency of the grating is the coupling coefficient, called .kappa.. This parameter is in turn dependent on a number of other parameters. For example, .kappa. increases as the difference in refractive indices between the materials employed on both sides of the grating increases. .kappa. increases also with increases in the overlap between the optical field created by the laser gain medium and the grating region. On the other hand, increases in the distance between the gratings and the active region of the laser cause corresponding decreases in the coupling coefficient. For example, in situations where the active regions are far away from the grating region, photons can hardly "see" the gratings, and .kappa. will remain low even if the grating is created between materials of large refractive index difference.
In all known DFB and DBR grating coupled laser structures, the value of .kappa. is almost always in the range of 50-100 cm.sup.-1. This is because any attempt of achieving a coupling coefficient significantly higher than 100 cm.sup.-1 (e.g., greater than 150 cm.sup.-1) will severely jeopardize the laser performance or even render the devices inoperable since the resulting optical loss caused by the strong grating coupling will be higher than the maximum optical gain which the active region can produce. This limit on the maximum value of the coupling coefficient also limits the smallest size of the device, which in turn limits device fabrication throughput by limiting the number of devices that can be formed on each wafer. With present grating coupled lasers, it is also not possible to reduce the output beam spot size to a size which matches the mode field of single mode optical fibers.